This invention relates to apparatus for inspecting the state of corrosion of fluid transfer pipes of large diameter especially cargo oil pipes used in an oil tanker.
Piping in an oil tanker poses the problems of maintenance and corrosion control.
An oil tanker typically comprises oil tanks for holding crude oil exclusively, permanent ballast tanks filled with only sea water, and tanks alternately filled with crude oil and sea water in every navigation. These tanks are connected with cargo oil pipes made of cast steel tubes of various diameters. Among those pipes, the cargo oil pipes in the main line measure about 600 to 700 mm in diameter, whereas those in the ballast line are about 400 to 500 mm in diameter.
Among factors which corrode such cargo oil pipes the main factors are chloride ions contained in seawater, inorganic sulfur compounds and organic sulfur compounds such as thioferromercaptans contained in crude oil, and sulfate ions contained in abundance in the sludge staying at the bottom of cargo oil pipes. During navigation, the cargo oil pipes are dipped in these corrosive liquids. Of these causes of corrosion, the effect of sludge is most manifest, and it is known that corrosion of a cargo oil pipe is generally initiated from the bottom part.
To inspect for corrosion, previously, ultrasonic waves or magnetic signals were applied from the outside of cargo oil pipes, and the reflected waves or eddy currents were checked for disturbance. This method, however, took much time and labor, and was difficult in complicated piping and facilities.
The simplest and most reliable method is for piping to be inspected by an inspector who gets into the cargo oil pipe and creeps along the piping to check for corrosion. The inside of the tank in a tanker is, however, extremely high or low in temperature due to thermal transfer from the ambient air or sea water, and is very narrow and thus provides a severe working environment, and, still worse, involves a risk of fatal danger due to the possibility of gas remaining in the pipe.
Besides, corrosion may be developing inside the metal of a pipe wall even if corrosion is not apparent from outside when the pipe wall is covered with a sludge deposit, in which case it is extremely difficult for the inspector, inside the pipe, to find such a concealed abnormality.